Have you ever thought of grilling a watermelon? I know it may seem completely backwards, but I promise, promise, promise you that it works. And it works well. If you're an American, and it's the summer, you probably have a watermelon and a grill somewhere in your house. And if you haven't combined the two yet, I invite you to do so now. This is probably the only time you'll get something off the grill and think of it as 'refreshing.' —Taylor Ayral
Lay your slices of pancetta on a sheet pan. Place the pan in the cold oven and set the temperature to 400 F. Bake until golden brown and crispy, about 15 minutes depending on how thinly your pancetta has been sliced. Allow to cool and crumble. Set aside.
Peel the jicama. Cut into 1/8" julienne (carefully cut into 1/8" slices, then place each 1/8 inch disk flat on the cutting board and cut across into 1/8" matchsticks). Set aside.
If using a gas grill, allow to preheat on high. If using charcoal, get a bed of hot, mature coals going. Meanwhile, remove the green and white peel from the watermelon with a serrated knife, so that only the red flesh remains. You don't have to use a seedless watermelon but if you don't you'll be spitting out some seeds as you eat this salad. Cut your melon into ~1/2" slices, and put them straight onto the grill.
Grill the melon for 3-5 minutes per side, enough to get a noticeably charred from the grating but still solid enough to be flipped easily. Once grilled on both sides, allow to cool enough to handle before sprinkling with 1/4 tsp of kosher salt and cutting into 1/2" cubes.
In a large salad bowl, combine the watermelon, jicama, arugula, and feta. Sprinkle with the balsamic and champagne vinegars, and the remaining salt and freshly ground black pepper. Add the pickled red onions and toss everything to combine.